1. Field of the Invention
The present invention-generally relates to a method and apparatus for switching the gray levels of a pixel in a liquid crystal display (LCD).
2. Description of the Related Art
While there are-several types of liquid crystal displays (LCDs), all LCDs operate on the same general principle. A liquid crystal material is placed in a sealed but light transmissive chamber and light transmissive electrodes are placed above and below the liquid crystal material. In one type of LCD utilizing what are called twisted nematic liquid crystals, when sufficient electric potential is applied between the electrodes, the liquid crystal molecules change their alignment. The change in alignment alters the polarization of light passing through the liquid crystal material. The chamber or cell essentially acts as a light shutter or valve, letting either a maximum, minimum, or intermediate levels of light through. These levels of light transmittance are called gray levels.
A matrix LCD structure is normally utilized for complex displays. A large number of very small independent regions, of liquid crystal material are positioned in a plane. Each of these regions is generally called a picture element or pixel. These pixels are usually arranged in rows and columns forming a matrix. Corresponding numbers of column and row electrodes are correlated with the rows and columns of pixels. An electric potential, also called a driving force, can therefore be applied to any pixel by selection of appropriate row and column electrodes and a desired graphic can be generated.
The amplitude of a driving force for a pixel depends on the gray level the pixel is going to present. FIG. 1 is a relational diagram between the light transmittance of a liquid crystal material and the driving voltage. Digitized by 3 bits, for example, the light, transmittance is represented by 8 gray levels, G0 to G7. Through the oblique line in FIG. 1, 8 driving forces, V0 to V7, for driving the liquid crystal material to respectively present the 8 gray levels under a static condition, can be determined. The conventional method for driving a pixel is to provide a driving force without consideration of dynamic switching. That is, if a pixel driver consecutively receives signals of gray level in a sequence of [G2, G0, G4, G5], for example, it consecutively provides the respective static driving voltages in a sequence of [V2, V0, V4, V5] to the pixel.
However, under dynamic conditions, the response rate for a liquid crystal material to change its light transmittance depends on the difference between the desired gray levels of the liquid crystal material in the previous and the current time frames. The smaller the difference the poorer the response rate. In other words, the switch between all-black and all-white is faster than a switch between intermediate levels. This results in bad graphic quality when an LCD displays highly dynamic pictures. Furthermore, the response rate also limits the maximum switching rate between picture frames and limits the application of an LCD for displaying TV programs. As shown in FIG. 2, when the response rate for gray level switching (the dash line in FIG. 2) is far behind the switch rate of the driving voltages (the solid line in FIG. 2), the pixel cannot present the current gray level.